Coupling devices (referred to as couplers) in general, such as for example Hybrid 3 dB couplers, are essential circuit components which are increasingly being used for high performance applications in such diverse circuits as RF mixers, amplifiers and modulators. In addition they can be used in a variety of other support functions such as the ones encountered in general RF signal and amplitude conditioning and error signal retrieval systems.
The expression “hybrid” in connection with couplers means an equal split of power between two (output) ports of the coupler with respect to an input port. Hence a 3 dB coupler is a “hybrid” since:10 log(Powerout/Powerin)=−3 dBPowerout/Powerin=10(−3/10)=0.5So the output power Powerout of one of the output ports is half (−3 dB) of the input power Powerin, the other half emerges from the other output port. If we consider FIG. 1 (to be explained in greater detail later on) and say that port P1 is the input port, then port P4 is said to be the coupled port and port P2 is said to be the direct port with half the input power being output from each of the output ports. Port P3 is said to be isolated from port P1. Note that the output at the coupled port will experience a phase shift dependent on the coupling length, while the output at the direct port will not experience a phase shift (with reference to the input supplied at the input port).
The use of couplers in the 1–5 GHz range though has been at the expense of large area of occupation required for such couplers and fabrication tolerance problems resulting from tight gap dimensioning for 3 dB coupling operation when implemented in PCB technology (PCB=Printed Circuit Board). More precisely, when implementing a coupler in PCB technology, it is necessary to accurately provide a gap between coupling lines of a coupler with the designed dimensions, since otherwise the coupler will not perform properly.
To address fabrication issues, narrow-band equivalents that compromise even more the size of the circuit such as branch line couplers have been utilized. Other alternatives such as SMD type (SMD=Surface Mounted Device) hybrid couplers have been used that offer better size ratios but are still quite large for future systems of small size with increased functionality. Often SMD component type couplers require additional external matching components to optimize their performance in terms of isolation and matching as well as amplitude and phase balance and therefore even further compromise the circuit area. Stated in other words, the provision of externally provided SMD components for matching purposes further increases the entire size of the coupler and requires additional soldering processes for soldering the externally provided SMD components. The increased use of SMD components increases costs and the use of soldering connections compromises the environmental friendliness and reduces the reliability of a manufactured subsystem module, since each solder connection represents a potentially source of errors.
Stripline technology has also been utilized for the design of high performance couplers but it suffers from the need to accommodate for larger volume/size for a given component inflicting additionally more materials costs.
Low loss performance can also be an issue especially in LNA designs (LNA=Low Noise Amplifier) as well as in high efficiency power amplification and linearisation applications. For such applications the dB on loss performance is a critical issue. Current designs offer typically 0.3 dB loss performance per coupler.
To rectify the above problems and address the performance requirements of future miniaturized circuit subsystems, wideband couplers in terms of isolation, matching and amplitude and phase balance are required that are additionally fabrication tolerance resistant and of much smaller size than its predecessors.
Size can be decreased by using an appropriate integration technology as well as a miniaturization circuit technique. Multilayer integrated circuits such as multilayer ceramic LTCC/HTCC (LTCC=Low Temperature Cofired Ceramics, HTCC=High Temperature Cofired Ceramics) technologies have been identified as a technology of great miniaturization potential since three dimensional design flexibility is combined with ceramic materials of high dielectric constant (∈). Loss performance is enabled by the careful choice of materials and circuit geometry as well as topology.
Isolation/matching and amplitude and phase balance performance can be optimized by using a suitable circuit technique or geometry.
FIG. 1 shows an equivalent circuit diagram of a conventionally known coupler. Basically, a coupling device consists of a pair of coupled lines 3a, 3b. Each line has two ports for inputting/outputting electrical and/or electromagnetic signals to be coupled. Thus, as shown in FIG. 1, the line 3a has ports P1, P2, while the line 3b has ports P3, P4. Each port P1 through P4 is terminated with a termination impedance Z0. In a 50 Ohms system, the value of Z0 is set to 50 Ohms. The lines 3a, 3b have equal length which is expressed in terms of the wavelength for which the coupler is designed. The parameter le° denotes the electric length of the coupler which is measured in degrees (°). For example, for the coupler shown in FIG. 1, the length is assumed to be λ/4, with λ being the center frequency of operation for which the coupler is designed. Thus, in such a case, a signal fed to the coupler at port P1 and used as a reference is coupled to the port P4 (coupled port) with its phase shifted (indicated by “−90°”). Port P3 is isolated from port 1, which means that no power reaches port P3 from port P1. The signal at port P2 (the direct port) is not shifted with reference to the signal input at port P1 as indicated by 0°. Note that in case of a 3 dB coupler as an example, the power input at port P1 is split between ports P2 (direct port) and P4 (coupled port) Nevertheless, other line lengths such as λ/2, or odd multiples of λ/4 such as 3λ/4 are possible. Also, the lines could have different lengths, while in such a case only the length of the lines over which the lines are facing each other represents an effective coupling length (electric length le in [°]). The coupler, i.e. the coupling lines, may be described in terms of the even and odd propagation modes of electromagnetic waves travelling there through and their respective characteristic impedances Zoo, Zoe and phase velocities υoe and υoo and the electric length le of the coupling lines.
In 3 dB coupling in a 50 Ohms system, one needs to design the lines to have impedance values Zoo and Zoe of 20.7 and 120.7 Ohms respectively. The above arrangement though assumes equal phase velocities for the even and the odd modes i.e. υoe=υoo.
If the phase velocities of the two modes (even and odd mode) are unequal, then isolation and matching at the centre frequency of operation suffers. More precisely, the undesired unequal phase velocities are typical for all transmission lines that are not strictly TEM (Transverse-Electro-Magnetic), often referred to as Quasi-TEM transmission lines such as for example a microstrip line. This is invariably the case with most couplers that use a pair of microstrip lines.
The problem of unequal phase velocities could be prevented by the use of true TEM transmission lines such as coupled striplines. However, in such a case at least one extra metallization layer is required which is not desired in terms of costs, involved.
FIG. 7 shows in a rough outline the difference between a stripline and microstrip arrangement, respectively. The left hand portion of FIG. 7 shows a stripline arrangement, while the right hand portion shows a microstrip arrangement. It is an important property of any two-conductor lossless transmission lines (coupling lines) placed in a uniform dielectric substrate (homogeneous and/or symmetrical substrate) that it supports a pure TEM mode of propagation. A common example of these types of lines is STRIPLINE, as shown in FIG. 7, left portion. However if a transmission line is placed in an inhomogenous (and/or non-symmetric) dielectric substrate it can no longer support fully-TEM propagation because the electromagnetic wave now propagates mostly within the substrate, but some of the wave is now able to propagate in air also. The most common example of this is MICROSTRIP also shown in FIG. 7, right portion. Stripline couplers are encased in a homogenous substrate where the electromagnetic fields of the coupler are confined within the substrate by the two ground planes (conductive layers) While for a microstrip line its electromagnetic propagation takes place mainly within the substrate (in fact most of the power propagates within the substrate), but some of the power propagates outside the substrate which is usually air.
FIG. 2 shows a cross section of a coupler (in microstrip arrangement) as represented in FIG. 1, while FIG. 2 shows coupled lines on the surface (FIG. 2a) or embedded (FIG. 2b) within a substrate as alternative microstrip arrangement implementations.
As shown in FIG. 2(a), the coupling device comprises a substrate 1 made of a dielectric material of a dielectric constant εr, a conductive layer 2 covering a first surface of said substrate 1 (the “bottom” side), and (at least) two lines 3a, 3b being provided electrically separated from each other at a second surface of said substrate 1 opposite to said first surface (the “top” side). Note that the same reference numerals as those used in FIG. 1 denote the like components such that a repeated explanation thereof is omitted. Said two lines 3a, 3b are laterally spaced apart from each other, with the amount of spacing (i.e. the width of a gap there between) adjusts the degree of electromagnetic coupling between said two lines. Although only two lines are shown, more than two lines may be used for coupling purposes dependent on the specific purpose for which the coupler is designed. Moreover, said conductive layer 2, in operation of the device, is connected to ground potential.
The coupler shown in FIG. 2(a) is generally known as an edge coupled coupling device, since coupling occurs between the elongated sides/edges in lengthwise direction of the lines facing each other (in a direction vertical to the drawing plane in FIG. 2(a)).
It is typical in such edge coupled microstrip line couplers that the odd mode velocity is higher than the even mode velocity i.e. υoo>υoe. Compensation techniques that improve isolation and matching and retain the amplitude and phase balance to good bandwidths have been dealt with previously. The main issue with these techniques is that such edge coupled couplers suffer from fabrication tolerances (gap dimension requirement such as small gap, constant over the entire length of the striplines), and therefore their use is not generally suggested.
The case when the even mode velocity is higher than the odd mode velocity (i.e. υoe>υoo) is a case that is encountered in the case of partially embedded broadside coupled microstrips (i.e. at least one coupling line being embedded).
Such a broadside coupled coupling device is illustrated in FIG. 2(b). Note that the same reference numerals as those used in FIG. 1 denote the like components such that a repeated explanation thereof is omitted. As shown, a broadside coupled coupling device comprises a substrate 1 made of a dielectric material of a dielectric constant ∈r, a conductive layer 2 covering a first surface of said substrate 1 (the “bottom” side), (at least) two electromagnetically coupled lines 3a, 3b being provided opposite to said first surface and being covered by at least one cover layer 4, 5.
The (at least) two lines 3a, 3b are arranged at different distances from said first surface of said substrate 1, with a difference between the distances in which said two lines 3a, 3b are arranged from said first surface of said substrate 1 is determined by a thickness of a first cover layer 4 covering a first line 3b of said at least two lines. As shown in FIG. 2(b), the first line 3b and second line 3a of said two lines are arranged such that they fully overlap each other in the cross-sectional representation. Nevertheless, this is not absolutely required and it is sufficient that they at least partly overlap each other. The amount of overlap (and of course the distance between the lines in “vertical” direction within the substrate) adjusts the degree of electromagnetic coupling between said at least two lines. Such an overlap is illustrated in FIG. 3B.
A second cover layer 5 is arranged to cover at least the second line 3a of said two lines. This means that as shown, the second cover layer 5 also covers the first cover layer. However, this is not absolutely required, while from a viewpoint of simplified production nevertheless desirable. The at least one cover layer 4, 5 is for example of the same material as said substrate 1. Moreover, said conductive layer 2, in operation of the device, is connected to ground potential.
Note that the arrangement shown in FIG. 2(a) may additionally be covered with a cover layer (not shown) so that either an edge coupled buried coupling device is obtained in case the cover layer is a dielectric material (e.g. the same as the substrate material), or an edge coupled coated coupling device is obtained in case the cover layer is e.g. a resist pattern.
FIG. 3 shows a further arrangement of a coupling device. FIG. 3B shows a coupling device in cross section with at least partly overlapping coupling lines as mentioned herein above. FIG. 3A shows a top view and/or layout view of the coupling device shown in FIG. 3B. Ports P1 and P2 are interconnected by the coupling line 3a which is arranged above the coupling line 3b interconnecting ports P4 and P3. Coupling line 3a and ports P1, P2 are illustrated in a differently hatched illustration as compared to coupling line 3b and ports P4 and P3.
Still further, the arrangements of FIGS. 2(a) and 2(b) and or FIG. 3 may be combined if e.g. more than two coupled lines are present in the coupling device. This means that for example edge coupled coupling lines may in turn be broadside coupled to one or more other coupling lines provided for in the arrangement.
Note also, that as the production technology for such devices, the multilayer integrated circuit technology which is assumed to be well known to those skilled in the art may be used so that a detailed description of the method for production of such devices is considered to be dispensable.
To the best of our knowledge there have not been suggested any techniques that compensates the velocity of the even and odd modes when the situation is encountered that the even mode velocity is higher than the odd mode velocity. Thus, in such a case, the above discussed problems inherent to coupling devices in connection with unequal phase velocities still remain.